Grass mowing machines known as zero turning radius (“ZTR”) mowers have independently powered drive wheels on each side of a frame. One drive wheel may be operated in a forward direction while the other drive wheel may be stopped or operated in reverse. Many ZTR mowers have a twin stick control system. A pair of control levers or sticks may be provided side-by-side, with each lever or stick controlling one of the drive wheels. When both levers or sticks are advanced together forwardly out of their neutral position, both drive wheels rotate forwardly to cause the mower to move forward. A ZTR mower may be steered by advancing one lever or stick more than the other.
Some ZTR mowers include a pair of individual integrated hydrostatic transmissions, each integrated hydrostatic transmission having a variable displacement pump and an independent wheel motor. Advantages of a pair of individual integrated hydrostatic transmissions include flexibility in the width of the mowing vehicle, and freeing up center space between the pair of transmissions.
In the past, each integrated hydrostatic transmission has included its own hydraulic fluid reservoir system. However, when a ZTR mower operates on a side slope, the fluid used in the integrated hydrostatic transmission for the downhill working drive wheel will increase in temperature significantly. Under those circumstances, the hydraulic fluid temperature for the downhill transmission may climb beyond the allowable fluid temperature.
One approach to reduce the hydraulic fluid temperature is to provide a hydraulic fluid cooler to provide extra cooling for each integrated hydrostatic transmission. However, the cooler and additional hydraulic lines can increase the cost of each integrated hydrostatic transmission significantly, and also may become plugged by debris in the mowing environment.
Alternatively, the surface area of an integrated hydrostatic transmission housing can be increased to help dissipate the heat and reduce the fluid temperature. An air flow device such as a fan also can increase the cooling capacity. However, this alternative is limited by cost and space constraints on a ZTR mower, and does not increase cooling capacity sufficiently under severe conditions.